1. Field of the Invention
The present invention generally relates to optical measurements in a controlled environment. In particular, the present invention relates to an air coupled probe for optical film measurements in a moist aqueous environment.
2. Description of the Related Art
Optical film thickness measurements are performed routinely in the semiconductor fabrication and machining processes to control film deposition and removal processes. In particular, chemical mechanical polishing (CMP) is used in semiconductor fabrication. In the CMP process, wafers containing thin films are polished in an aqueous environment to planarize the surface. The CMP process is often performed as a multistep process during which the wafers being polished are kept wet. It is desirable to measure the thickness of the material so that the wafers are not over polished and so that surface planarity may be gauged. Presently, the wafers are first removed from the polishing tool, cleaned and dried before film measurements may be made. Thus, there is a significant delay between the polishing operation and the measurement of the results of the polishing operation.
There is also excessive handling of the wafers between the polishing tool and the measurement area which results in significant cost.
It is desirable to measure the film thickness as soon as possible after the polishing process. However, this has not been practical to measure the film thickness immediately after the polishing process, because the wafer is placed in a rinsing tank and rinsed with water. It is very difficult to obtain an accurate thin film measurement in this environment.
Optical measurements are most easily performed in air. In other words, the optical beam used to make the measurement should pass through air directly to the surface film being measured.